In a selective call communication system a particular receiver is rendered operative when the carrier wave signal applied thereto contains a certain word or set of words to which the decoder is such receiver is designed to respond. The term "word" is intended to include digital data and analog data such as tones. In systems involving voice communication, the words are transmitted to unsquelch the receiver, whereupon the operator at the transmitter can speak into his microphone and the possessor of the associated receiver will hear the voice communication. Alternatively, the system may involve nonvocal communication, wherein the receiver emits an alerting signal, such as a tone, when the proper words are received.
It has been proposed to use selective call capability in an automatic identification system. In such system, each station, which may be a vehicle, is furnished with a transmitter and an associated encoder to enable the driver to communicate with the base station and also to identify himself without so stating. When the vehicle driver wishes to communicate with the base station, he operates his push-to-talk switch and speaks into his microphone. With automatic identification capabilities, the encoder in his equipment generates a code representing that encoder, which code may be a sequence of tones modulated onto the carrier wave. An identification signal, by way of a suitable display for example, apprises the base station operator of the identity of the station transmitting.
In systems incorporating an automatic identification feature, there are likely to be many mobile stations communicating with the base station, thereby causing frequent transmission of automatic identification signals, and the increased likelihood of falsing the base station. For example, in a system in which the transmitter at each mobile station produces a code including a three-tone sequence, the base station may display a three-digit number corresponding to that mobile station. Two such mobile stations may transmit nearly simultaneously so that the codes overlap. If the strength at the receiver of the signal containing the later starting code is substantially greater than that of the first code, a sequence of four or five or even six tones, depending upon the precise relationship between the two sequences, could be applied to the decoder at the base station. The decoder may respond, for example, to the first two tones of the first code and the first tone of the second code, and therefore cause erroneous information to be displayed. If the strength of the two codes is nearly the same, then the receiver will respond to a hybrid of the overlapping tones. For example, if the last tone in the first code overlaps the first tone in the second code, the decoder will respond to a sequence containing the first two tones and a hybrid of the two overlapping tones, again causing erroneous information to be displayed. A secondary difficulty is the possibility that, for one reason or another, less than the expected number of tones is transmitted to cause an erroneous number to be displayed.
In prior systems having a display that supplied information as the result of a sequence of words, the digits are reproduced on the display just as quickly as they are received. Normally this would not present a significant problem since the lag between the tones is on the order of just 50 milliseconds or less, so that the operator cannot perceive that there is a lag between the occurrence of the display of the several digits. However, in the system described above, the presence of an improper code to such a decoder will result in an improper display.